Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
The performance of flash memory devices needs to increase as the performance of computer systems increase. For example, a flash memory transistor that can be erased faster with lower voltages and have longer retention times could increase system performance.
Amorphous silicon (a-Si)-based thin film transistors (TFT) have been used to improve transistor performance. However, these transistors have undesirable short channel effects and randomness in device characteristics. This is largely due to the randomness in polysilicon grain size and grain boundaries that exist in TFT devices. For example, in sub-75 nm feature sizes, this can result in a very large variation in device characteristics making the stacked cell approach extremely challenging for mass manufacture.
Silicon carbide (SiC) substrates have been used in power devices due to the higher bandgap over silicon. Wide bandgap material like SiC substrates have very low intrinsic carrier concentration and thermal generation scales directly with the intrinsic carrier concentration. Therefore, junction leakage currents in SiC substrate devices are very low.
However, SiC substrates suffer numerous problems. For example, wafer sizes in excess of four inches that have high quality and low cost are difficult to achieve. Additionally, the defect densities are unacceptable and the substrates suffer from poor carrier mobility for high speed switching.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a higher performance flash memory transistor that does not have serious scaling issues.